Spring of Storms

NASA/JPL

This spring’s weather can only be described as extreme, breaking records for drought, wildfire and flooding across the U.S. and costing billions of dollars in damages. According to the National Climate Data Center (NCDC), the 2011 spring season has had the most combined wet and dry weather extremes on record since 1910. Texas experienced its worst drought on record, the Lower Mississippi River saw a massive record-breaking flood, the worst fire season to date ripped through southern states and tornado activity spiked. Over the course of this past March, April and May, 46% of the nation had abnormally wet or dry conditions, the most extreme in over a century.

So what’s causing all this severe weather? In part, it is likely influenced by winter La Niña. Typically, La Niña occurs in the Eastern Pacific when waters around the equator cool to several degrees below average. This causes unusually dry winter weather in the south and unusually wet weather in the Midwest, altering the course of the jet stream so that the winter storm track bypasses the southern states and shifts over the Midwest instead. The effects of La Niña commonly begin to dissipate in early spring, but this spring the effects were more extreme, and dipped into the spring season with full force.

What’s more, the jet stream was lower this year than in past La Niña events. As a result, the usual upheaval of weather conditions under La Niña became more pronounced. Winds were much stronger along the jet stream. The lower position of the jet stream brought abnormally cold weather to the Pacific Northwest, while in the Midwest spring was much warmer than usual. The mixing of colder, deeper oceanic currents was reduced by the change in air speed and temperature moving over the ocean, leading to warmer ocean surface temperature and increased evaporation of seawater into the atmosphere, in turn leading to increased precipitation and storm events over the states.

Beyond the influence of La Niña, many wonder if this year’s extreme spring weather has any link to global climate change. Unfortunately, the answer isn’t clear. As Dr. Jeff Masters explains on his weather blog, “This spring’s unusual precipitation pattern–wet in the Northern U.S., and dry in the South–does fit what we’d expect from a natural but unusually long-lived winter La Niña pattern. However, it also fits the type of precipitation pattern climate models expect to occur over the U.S. by the end of the century due to human-caused warming of the climate.”

So can this spring be seen as a preview of what is to come for a world undergoing a shift in climate? Not exactly. Jessica Rennells, a climatologist with the Northeast Regional Climate Center at Cornell University, told the Times Union, “You cannot take any events in a single spring and say that is definitely a result of climate change… But you can say that some events, like more intense storms from more moisture in the atmosphere, are what we would expect in a changing climate.”

While the crazy weather of this past spring does raise concerns for the future, it is too soon to tell if this type of weather will become the norm as global warming increases. But it’s a very real possibility.